does hot water freeze befor cold water ,

John Felciano

is right...This discussion comes up every couple years.I actually had to test it out at home in the freezer to believe it for myself.

Water that HAD been heated and than returned to ambient will freeze before cold water of the same temperature.I saw it with my own eyes....

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ed

hot water freeze befor cold?

i do not think that hotwater freezes befor cold, any boby have the prove that i can provide to win this discussion. thanks inadvance

Tony_4

I rememeber this question on here 3 years ago

The answer is no but water that is heated will freeze faster than water that hasnt been heated because it contains less dissolved gases.
The reason a lake will freeze on top first isnt because the water on top is warmer but because it is less dense.

DanHolohan

Here's the link to the answer:

http://www.physlink.com/Education/AskExperts/ae7.cfm

[Deleted User]

well according to the PhD, i guess i'm wrong

but my experiances over twenty plus years tell me i've repaired many more hot water pipes that froze and ruptured sitting right next to cold water pipes that did neither. altho i like the first four words he uses in dan's link...all things being equal....maybe they are not equal in the real world.

JimGPE_3

That's an old engineer's trick!

Change the problem to one you CAN solve! I love it! ("Assume a fire extenguisher")

Nah, just kidding. Interesting that water that ONCE was hot froze faster. I have this discussion with my son every so many months - but neither of us ever cared enough to do the experiment! One of these days I will.

John Abbott

Freezing

Cold water will freeze before hot water.The laws of physics are irrefutable.
However having said that let me clarify.Water releases dissolved gasses when heated and will cause expansion dammage in pipes faster than cold water because of the lack of the cushioning effect of the entrained gases ie: dissolved air.Also as was said in an earlier post it will also freeze faster than previously unheatead water due to the lack of the insulating effect of the dissolved air.

John

antman

I'm with Gill,

Almost all freez-up calls I go on are HOT WATER lines in the same chase as the cold water, I may not be a Doctor but i've seen this many many times, not all the time( outside hose bib not shut-off) but did I say many many.

:-) Ant

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Rick Kelly

Physics at work

Think about it. Heat goes to cold and the greater the delta T, the faster the process will be. Consider pressure in the case of a closed pressure loop (Boyle's Law).

Frank Cheng

his example may be different from what we are talking about here

He is talking about hot water that is still hot vs. cold water and see which freezes faster. In most of our situations, we have cooled-down hot water vs. cold water that is never heated before. In this situations, most of the time, the cooled-down hot water freezes faster according to many people's experiences, including myself. Probably, when being heated up, water lost gasses and some minernals (like calcium) in it. I think the minernals in water will lower the liquid's freezing temperature, like adding salt to water.

Frank

Frank Cheng

another possible contradictary

That Ph.D. of chemistry may be talking about pure water or distill water that contains no gasses or mineral dissolves, not our city water that is not pure chemically speaking. Did anybody ask him what kind of water he used for his freezing experiment?

Frank

Unknown

Here's a good explanation...

http://www.weburbia.com/physics/hot_water.html

Written Nov 4, 1998 by Monwhea Jeng (Momo)
Department of Physics, University of California, 93106

Can hot water freeze faster than cold water?
I. Yes -- a general explanation
II. History of the Mpemba Effect
III. More detailed explanations
IV. References
I. Yes -- a general explanation
Hot water can in fact freeze faster than cold water for a wide range of experimental conditions. This phenomenon is extremely counter- intuitive, and surprising even to most scientists, but it is in fact real. It has been seen and studied in numerous experiments. While this phenomenon has been known for centuries, and was described by Aristotle, Bacon, and Descartes [1-3], it was not introduced to the modern scientific community until 1969, by a Tanzania high school student named Mpemba. Both the early scientific history of this effect, and the story of Mpemba's rediscovery of it, are interesting in their own right -- Mpemba's story in particular provides a dramatic parable against making snap judgements about what is impossible. This is described separately below.

The phenomenon that hot water may freeze faster than cold is often called the Mpemba effect. Because, no doubt, most readers are extremely skeptical at this point, we should begin by stating precisely what we mean by the Mpemba effect. We start with two containers of water, which are identical in shape, and which hold identical amounts of water. The only difference between the two is that the water in one is at a higher (uniform) temperature than the water in the other. Now we cool both containers, using the exact same cooling process for each container. Under some conditions the initially warmer water will freeze first. If this occurs, we have seen the Mpemba effect. Of course, the initially warmer water will not freeze before the initially cooler water for all initial conditions. If the hot water starts at 99.9 degrees Celsius, and the cold water at 0.01 degrees Celsius, then clearly under those circumstances, the initially cooler water will freeze first. However, under some conditions the initially warmer water will freeze first -- if that happens, you have seen the Mpemba effect. But you will not see the Mpemba effect for any initial temperatures, container shapes, and cooling conditions.

This seems impossible, right? Many sharp readers may have already come up with a common proof that the Mpemba effect is impossible. The proof usually goes something like this. Say that the initially cooler water starts at 30 degrees Celsius and takes 10 minutes to freeze, while the initially warmer water starts out at 70 degrees Celsius. Now the initially warmer water has to spend some time cooling to get to get down to 30 degrees Celsius, and after that, it's going to take 10 more minutes to freeze. So since the initially warmer water has to do everything that the initially cooler water has to do, plus a little more, it will take at least a little longer, right? What can be wrong with this proof?

What's wrong with this proof is that it implicitly assumes that the water is characterized solely by a single number -- the average temperature. But if other factors besides the average temperature are important, then when the initially warmer water has cooled to an average temperature of 30 degrees Celsius, it may look very different than the initially cooler water (at a uniform 30 degrees Celsius) did at the start. Why? Because the water may have changed when it cooled down from a uniform 70 degrees Celsius to an average 30 degrees Celsius. It could have less mass, less dissolved gas, or convection currents producing a non-uniform temperature distribution. Or it could have changed the environment around the container in the refrigerator. All four of these changes are conceivably important, and each will be considered separately below. So the impossibility proof given above doesn't work. And in fact the Mpemba effect has been observed in a number of controlled experiments [5,7-14]

It is still not known exactly why this happens. A number of possible explanations for the effect have been proposed, but so far the experiments do not show clearly which, if any, of the proposed mechanisms is the most important one. While you will often hear confident claims that X is the cause of the Mpemba effect, such claims are usually based on guesswork, or on looking at the evidence in only a few papers and ignoring the rest. Of course, there is nothing wrong with informed theoretical guesswork or being selective in which experimental results you trust -- the problem is that different people make different claims as to what X is.

Why hasn't modern science answered this seemingly simple question about cooling water? The main problem is that the time it takes water to freeze is highly sensitive to a number of details in the experimental set- up, such as the shape and size of the container, the shape and size of the refrigeration unit, the gas and impurity content of the water, how the time of freezing is defined, and so on. Because of this sensitivity, while experiments have generally agreed that the Mpemba effect occurs, they disagree over the conditions under which it occurs, and thus about why it occurs. As Firth [7] wrote "There is a wealth of experimental variation in the problem so that any laboratory undertaking such investigations is guaranteed different results from all others."

So with the limited number of experiments done, often under very different conditions, none of the proposed mechanisms can be confidently proclaimed as "THE" mechanism. Above we described four ways in which the initially warmer water could have changed upon cooling to the initial temperature of the initially cooler water. What follows below is a short description of the four related mechanisms which have been suggested to explain the Mpemba effect. More ambitious readers can follow the links to more complete explanations of the mechanisms, as well as counter- arguments and experiments that the mechanisms cannot explain. It seems likely that there is no one mechanism that explains the Mpemba effect for all circumstances, but that different mechanisms are important under different conditions.

Evaporation -- As the initially warmer water cools to the initial temperature of the initially cooler water, it may lose significant amounts of water to evaporation. The reduced mass will make it easier for the water to cool and freeze. Then the initially warmer water can freeze before the initially cooler water, but will make less ice. Theoretical calculations have shown that evaporation can explain the Mpemba effect if you assume that the water loses heat solely through evaporation [11]. This explanation is solid, intuitive, and evaporation is undoubtedly important in most situations. However, it is not the only mechanism. Evaporation cannot explain experiments which were done in closed containers, where no mass was lost to evaporation [12]. And many scientists have claimed that evaporation alone is insufficient to explain their results [5,9,12].
Dissolved Gasses -- Hot water can hold less dissolved gas than cold water, and large amounts of gas escape upon boiling. So the initially warmer water may have less dissolved gas than the initially cooler water. It has been speculated that this changes the properties of the water in some way, perhaps making it easier to develop convection currents (and thus making it easier to cool), or decreasing the amount of heat required to freeze a unit mass of water, or changing the boiling point. There are some experiments which favor this explanation [10,14], but no supporting theoretical calculations.
Convection -- As the water cools it will eventually develop convection currents and a non-uniform temperature distribution. At most temperatures, density decreases with increasing temperature, and so the surface of the water will be warmer than the bottom -- this has been called a "hot top." Now if the water loses heat primarily through the surface, then water with a "hot top" will lose heat faster than we would expect based on its average temperature. When the initially warmer water has cooled to an average temperature the same as the initial temperature of the initially cooler water, it will have a "hot top," and thus its rate of cooling will be faster than the rate of cooling of the initially cooler water at the same average temperature. Got all that? You might want to read this paragraph again, paying careful distinction to the difference between initial temperature, average temperature, and temperature. While experiments have seen the "hot top," and related convection currents, it is unknown whether convection can by itself explain the Mpemba effect.
Surroundings -- A final difference between the cooling of the two containers relates not to the water itself, but to the surrounding environment. The initially warmer water may change the environment around it in some complex fashion, and thus affect the cooling process. For example, if the container is sitting on a layer of frost which conducts heat poorly, the hot water may melt that layer of frost, and thus establish a better cooling system in the long run. Obviously explanations like this are not very general, since most experiments are not done with containers sitting on layers of frost.
Finally, supercooling may be important to the effect. Supercooling occurs when the water freezes not at 0 degrees Celsius, but at some lower temperature. One experiment [12] found that the initially hot water would supercool less than the initially cold water. This would mean that the initially warmer water might freeze first because it would freeze at a higher temperature than the initially cooler water. If true, this would not fully explain the Mpemba effect, because we would still need to explain why initially warmer water supercools less than initially cooler water.

In short, hot water does freeze sooner than cold water under a wide range of circumstances. It is not impossible, and has been seen to occur in a number of experiments. However, despite claims often made by one source or another, there is no well-agreed explanation for how this phenomenon occurs. Different mechanisms have been proposed, but the experimental evidence is inconclusive. For those wishing to read more on the subject, Jearl Walker's article in Scientific American [13] is very readable and has suggestions on how to do home experiments on the Mpemba effect, while the articles by Auerbach [12] and Wojciechowski [14] are two of the more modern papers on the effect.

[Deleted User]

wow Noel

you made my brain hurt...good read tho,, i archived that one.

John Abbott

Heat goes to cold?

as we have always been taught?
If that is the case please explain the phenomenon which occurs when I quench the cherry red end of apiece of hard temper copper in water and the heat can be felt rushing up the tubing away from the colder water? We do this to allow for plumbing of risers in imperfectly aligned fittings. Its not a perfect world alas.

John

[Deleted User]

i did an experiment last night

i boiled some water, allowed it to cool, put it in a glass, also took a glass and put the same amount of cold water in it. put them in the freezer for an hour or so. they werent completly frozen, so when i tipped the glasses, the one with cold water in it had quite a bit of water slip out from under the ice, but the previously heated water had less slip out from under the ice. noticibly less...to me that mean't the previously heated water froze faster....interesting to note also, i left the glasses on the counter before going to bed. when i woke up the glass with the once hot water was clear. the glass with the cold water had lots of bubbles stuck to the sides. i think its called surface tension, but i'm not really sure what it is.